POPULATION FACTORS AND SELECTED POPULATION PROBLEMS 



357 



when diluted 1:400,000 or even 1:800,000 

 times. At these dilutions it is not probable 

 that this factor is affecting growth by fur- 

 nishing food material. 



There exist other possibilities, some of 

 which have been explored by Allee and his 

 group. The increase in growth is not 

 caused, for example, by a change in the to- 

 tal salt content of the water, for this does 

 not vary in the experiments. It is not 

 caused by differences in acidity or oxygen- 

 ation, nor, so far as careful quantitative 

 analyses have revealed, by changes in the 

 chemical elements present. It is also a pos- 

 sible hypothesis that the fish population re- 

 moves some harmful substance from the 

 water. This sort of conditioning will be 

 discussed shortly. 



The most recent statement summarizing 

 the status of this problem is that of Allee, 

 Finkel, and Hoskins (1940, p. 436): 



"Evidence available in 1936 indicated that 

 the caloric food value of these minute particles 

 is the major growth promoting factor in our 

 conditioned medium and we have no reason 

 to modify this conclusion. That this is not 

 the only growth accelerating factor which 

 is acting is indicated (c) by the relatively 

 beneficial effect of water contaminated only by 

 the presence of starved conditioning fish; (h) 

 by the growth promoting power shown by 

 filtered conditioned water and, most significant, 

 ( c) by the effect of protein extracts prepared 

 from the surface of other goldfish. It seems 

 his;hly significant that these latter will promote 

 additional growth over that shown by controls 

 when one part is dissolved in 400,000 parts of 

 water. The caloric value of such a medium 

 must be very small and no mechanism is known 

 whereby fish might concentrate such a dis- 

 solved food substance until it would have 

 significant caloric food value." 



Commenting upon the ecological signif- 

 icance of this investigation, Allee (1938, 

 pp. 97-98) says, "Whatever the explana- 

 tion, we are certain of the facts, and we 

 know that we have demonstrated a device 

 such that if in nature one or a few fish in 

 a group find plenty of food, apparently 

 without willing to do so they regurgitate 

 some food particles which are taken by 

 others, a sort of automatic sharing. Again, 

 in water that changes rapidly, such stag- 

 nant-water fishes as goldfish, if present in 

 numbers, are able to condition their en- 

 vironment, perhaps by the secretion of 

 mucus, so that it becomes a more favorable 

 place in which to live and grow." 



This analysis of habitat alteration by 

 goldfishes contributes to our survey of the 

 conditioned medium problem primarily by 

 illustrating two aspects of the conditioning 

 process: (1) partial distribution of avail- 

 able food, and (2) hberation of a growth- 

 promoting, or some other needed substance, 

 to the environment. 



The " Allelocatalytic Phenomenon" in Pro- 

 tozoan Populations. The favorable condi- 

 tioning of the habitat by its population 

 through the elaboration of a growth promot- 

 ing substance is suggested by the long 

 series of studies dealing vvath "allelocataly- 

 sis" and conducted by many investigators 

 working with a number of species of pro- 

 tista. Their work merits a short review. 



In 1921 T. B. Robertson published a re- 

 port on the population growth rate of two 

 ciliate protozoans, Enchehjs and Colpoda. 

 He found that when either of these two 

 forms was introduced into a small volume 

 of fresh culture medium, the fission rate 

 per organism was increased by a certain 

 amount of crowding. Specifically, two in- 

 fusoria in the same environment repro- 

 duced from 2.5 to ten times as fast as did 

 isolated individuals. This difference was 

 significant statistically. Robertson called 

 these events and their interpretation "alle- 

 locatalvsis." It is important to distinguish 

 carefully between the fact of allelocatalysis 

 just outlined and the theory to follow. 



In explaining his observations, Robertson 

 stated: "During nuclear division each nu- 

 cleus retains the charge of autocatalyst with 

 which it was provided, and adds to it dur- 

 ing the course of nuclear synthesis. At each 

 division the autocatalyst is shared between 

 the nuclear substance and the surrounding 

 medium in a proportion determined by its 

 relative solubility and by its affinity for the 

 chemical substances within the nucleus. 

 The mutually accelerated or allelocatalytic 

 effect of contiguous cells is due to each 

 cell's losing less of the autocatalyst to the 

 medium because of the presence of the 

 other" (Allee, 1931, p. 166). 



The evidence in support of the allelo- 

 catalytic theory seemed feeble. Whether for 

 this reason or another, a number of inves- 

 tigators started working along similar lines, 

 using various flagellate and ciliate species. 

 Most of these early workers reported nega- 

 tive results, that is, data showing that the 

 isolated organisms reproduced as fast as or 

 faster than the grouped organisms. None of 



